It's late autumn: temperatures are dropping, the open water is cooling and sea ice is re-growing in the Arctic Ocean. Polar bears are gathering here at Cape Churchill on the shore of Hudson Bay, waiting for the sea ice to freeze here, too, so they can go out on the sea ice and catch seals. They will hunt at the breathing holes that seals scratch through the sea ice or near naturally occurring cracks formed by the action of waves and winds. It is the normal cat and mouse game of bears and seals.
I look into the eyes of the bears here, just a few feet away, and I feel that our roles are reversed in a strange way. The bears should be floating on the sea ice at this time of year. Instead, it's me who is floating -- though over the tundra in an elevated buggy for protection from these fierce beasts.
Hudson Bay is historically ice-free in summer and early fall. Satellites in orbit since 1979 tell us that sea ice on the bay starts to melt in early May, and it melts away completely by late July. Re-growth begins in early November, and the bay is covered in ice in late December. Over the satellite era, the onset of the summer melt has shifted earlier and the freeze-up has slipped later in time, so now the ice-free season has expanded by more than a month.
Sea ice on the Arctic Ocean has experienced dramatic changes as well. The areal extent of the ice in late summer has decreased by about 1/3. The record sea ice minimum was set this September, shattering the previous record and surprising many sea ice experts once again. Most of us expected the sea ice to recover after the previous record low in September 2007. Instead, the last six years are the six lowest years on record. The wintertime sea ice cover is decreasing too, but much less quickly. If humans continue to burn fossil fuels at expected middle-of the-road or higher rates, climate model projections foretell Arctic sea-ice cover in summer by the end of this century will be a tiny remnant of the 20th century extent, and the Arctic could go virtually sea-ice free as early as a matter of decades.
One of the guests at the lodge here asked me why the Arctic sea ice is shrinking so fast and yet the Antarctic sea ice has expanded slightly over the satellite era. It is a good question. Observations and models show the Arctic sea ice loss is pretty clearly proportional to global warming of Earth's surface. The extent of the Antarctic sea ice depends on the competition between the cold surface winds blowing off Antarctica and the considerable amount of heat transported southward by the ocean.
Associated with global warming, the poles receive more precipitation and the locations where sea ice grows and melts shifts. When the ocean surface receives a greater amount of fresh water, the surface waters become less dense. Such freshening has been effective at inhibited oceanic heat from reaching the sea ice in the Antarctic, which has won out over surface air warming so far.
In contrast, freshening in the Arctic hasn't had the same effect because little oceanic heat reaches sea ice on the Arctic Ocean. Oceanic heat transport to the Arctic Ocean is obstructed by the surrounding continents, so it can only enter through narrow straits. With less heat reaching the Arctic Ocean, sea ice has historically been thicker on the Arctic Ocean compared to the Southern Ocean -- a condition that has changed over the past few years because rising air temperatures have won out over historically weaker oceanic heat transport.
Global warming can have surprising impacts. I hope I have explained that there is no contradiction for global warming to cause dramatic sea ice loss in the Arctic and a slight expansion in the Antarctic. There is no doubt in my mind that the implications for polar bears habitat loss are serious and sobering.
Photos via explore.org